THE DISSIPATION OF MAGNETOHYDRODYNAMIC TURBULENCE RESPONSIBLE FOR INTERSTELLAR SCINTILLATION AND THE HEATING OF THE INTERSTELLAR-MEDIUM

I reexamine the issue of heating of the interstellar medium via the damping of plasma irregularities responsible for interstellar scintillations of radio sources. The primary innovation of this paper is to consider the effects of recent observational results on the host plasma of the irregularities, as well as developments in the theory of MHD waves. I assume that the host plasma for the density irregularities causing interstellar scintillation is either extended envelopes of bright H II regions which have been revealed by low-frequency recombination lines, or the warm ionized medium of the McKee-Ostriker model. With the plasma characteristics approximate to these regions, one can calculate the rate at which the irregularities damp, and therefore the rate at which energy is input to the interstellar medium. The damping mechanisms I consider are linear Landau damping, ion-neutral collisional damping, nonlinear steepening of wave packets, a parametric decay instability, and nonlinear Landau damping. One of the principal parameters determinig the heating rate is the outer scale of the power-law turbulence, a quantity which is subject to observational constraint. The heat input from all damping mechanisms can be accommodated by the region containing the irregularities (the "fluctiferous medium" or "fluctifer") if it has characteristics similar to the extended H II envelopes. Damping of hydromagnetic waves could quite possibly be the dominant heating mechanism for this part of the interstellar medium. Depending on the outer scale of the turbulence, the dominant heating mechanism of this sort in the H II region envelopes is ion-neutral collisional damping, linear Landau damping, or dissipation associated with a parametric decay instability of large-amplitude MHD waves. The damping of turbulence would provide excessive heat to the McKee-Ostriker warm ionized phase, unless the outer scale of the turbulence is greater than about 10(17) cm. In any case wave dissipation would almost certainly be an important heating mechanism in this phase. The conclusion of this paper is that dissipation of the turbulence responsible for interstellar scintillation need not be a problem for our understanding of the interstellar medium, and that the fluctiferous medium is most plausibly identified with the extended envelopes of H II regions.